Allow more control over the text layouting process in QML.

Give access to every text line through a hook, this gives the
opportunity to position and resize a line as it is being laid out.
It is then possible to lay out the text in columns or around other
objects.

Task-number: QTBUG-21367
Change-Id: I56dc0c1c4b575dc06360c135098024d0324d3656
Reviewed-on: http://codereview.qt-project.org/5351Reviewed-by: Yann Bodson <yann.bodson@nokia.com>
Sanity-Review: Yann Bodson <yann.bodson@nokia.com>